1. Field of the Invention
The present invention relates to resin gears and throttle devices incorporating the resin gears. The present invention also relates to methods of manufacturing the resin gears and apparatus for manufacturing the resin gears
2. Description of the Related Art
Japanese Laid-Open Patent Publication No. 2004-84503 discloses a known electronically controlled throttle device having a resin gear fixed to a throttle shaft that has a throttle valve mounted thereto. The resin gear has a gear body having an outer peripheral portion formed with a toothed portion and a non-toothed portion. The resin gear is molded by injecting molten resin into a molding die having injecting gates provided on the side of the gear body. Japanese Laid-Open Patent Publication No. 2004-358665 discloses a resin gear having a toothed portion formed throughout the circumferential length of the outer periphery of the resin gear.
The resin gear disclosed in the Publication No. 2004-84503 has a concave portion disposed radially inwardly of the boundary region between the toothed portion and the non-toothed portion. A welded portion is formed at a merging point between the flow of molten resin detouring the concave portion and passing through the side of the tooted portion and the flow of molten resin detouring the concave portion and passing through the side of the non-toothed portion merge. The presence of the welded portion causes degradation in the strength of the teeth of the toothed portion and the molding accuracy of the toothed portion. Presumably, this degradation is caused because the distance of the flow of molten resin detouring the concave portion and passing through the side of the non-tooted portion from the corresponding injection gate to the welded portion is shorter than the distance of the flow of molten resin detouring the concave portion and passing through the side of the tooted portion from the corresponding injection gate to the welded portion. In the case of the resin gear disclosed in the Publication No. 2004-358665, this problem may not occur because a non-toothed portion is not provided.
Therefore, there is a need in the art for improving the strength and the molding accuracy of a resin gear that has a concave portion.